Variable resistive film attenuator

ABSTRACT

A continuously variable resistive film attenuator (often called &#39;&#39;&#39;&#39;card attenuator&#39;&#39;&#39;&#39; is provided, having distributed shunt and series film portions for use from D.C. above 8 GHz. It has a low minimum loss. It stays matched over its entire range. In one version it has substantially no incremental phase shift with changing attenuation. The resistive film is enclosed inside a slidable conductive sleeve which is moved to expose only that part of the attenuator which is inserted into the circuit, the unused part thus being in a field-free region where it can have no effect, while the inserted part of the shunt film is connected to ground through a continuous ground contact which is pressed down on the edge of the shunt portion by a multiple finger spring arrangement. In one version, the electrical length between input and output is maintained constant by shortening a conductive portion of the attenuator as the resistive portion is lengthened so that there is no phase shift between input and output as the attenuation is varied.

ergiried Jan. 15, 1974 1 VARIABLE RESKSTIVE FILM ATTENUATOR [75]Inventor: Dietrich E. Bergiried, Potomac, Md.

[73] Assignee: Weinschel Engineering Co. Inc.,

Gaithersburg, Md.

[22] Filed: June 26, 1972 [21] Appl. No.: 266,366

Primary Examiner-Paul L. Gensler Attorney-Max L. Libman 5 7] ABSTRACT Acontinuously variable resistive film attenuator (often called cardattenuator is provided, having distributed shunt and series filmportions for use from DC. above 8 61-12. It has a low minimum loss. Itstays matched over its entire range. In one version it has substantiallyno incremental phase shift with changing attenuation. The resistive filmis enclosed inside a slidable conductive sleeve which is moved to exposeonly that part of the attenuator which is inserted into the circuit, theunused part thus being in a field-free region where it can have noeffect, while the inserted part of the shunt film is connected to groundthrough a continuous ground contact which is pressed down on the edge ofthe shunt portion by a multiple finger spring arrangement. In oneversion, the electrical length between input and output is maintainedconstant by shortening a conductive portion of the attenuator as theresistive portion is lengthened so that there is no phase shift betweeninput and output as the attenuation is varied.

7 Claims, 10 Drawing Figures PMENTEU JAH sum 1 BF 2 VARIABLE RESISTIVEFILM ATTENUATOR BACKGROUND OF INVENTION This invention relates tovariable attenuators for use in high frequency coaxial lines. Theattenuator consists of a resistive film which has distributed series anddistributed shunt elements. An object of the invention is to provide acontinuously variable attenuator which has a substantially flatfrequency response over a wide range of frequencies from DC. to, forexample 8 GHz. Other objects are to provide an attenuator which staysmatched independent of attenuation position, has in one version small orno incremental phase shift with changing attenuation and essentiallyzero loss in the minimum attenuation position.

The basic problem with which the present invention is concerned can beillustrated by considering the limitations of some of the resistivevariable attenuators now in use such as the variable T pad (U.S. Pat.No. 3,184,694) and the variable card attenuator (US. Pat. No.3,157,846). The variable T-pad consists of three adjustable resistanceelements in T formation, two of which are connected in series with theinput and output center conductors and are therefore called seriesresistors. The third resistor is called the shunt resistor connectedfrom the common point to ground. The attenuation of the T pad attenuatoris varied by simultaneously moving contact points along each of theresistors thus changing the values of the three resistors in such a waythat the T-network maintains a constant input and output impedance andat the same time pro vides the desired attenuation. However, theindividual resistor elements will stay constant in value only below thefrequencies where the wavelength is long compared to the physicaldimensions of the resistor. When this condition fails, the value of theresistor and therefore the attenuation of the unit will change withfrequency. This limits the usable frequency range and becomes especiallyimportant for values of attenuation greater than 20 dB. Depending on theattenuation setting, a shorter or longer part of the series resistors isconnected between input and output, and since the length of thisposition is changing with the attenuation adjustment, the phase shiftchanges also in the simple version.

Variable card attenuators have an attenuation element which usuallycomprises a dielectric substrate supported within the coaxial structureand coated with one or more layers of resistive film. Such attenuatorsare usually subject to the following difficulties: (a) A minimum loss ofat least 3dB because of shunting effect of the unused portion of theattenuator element in parallel with the output; (b) Not matched in allpositions, only compromised match; (c) Phase shift with changingattenuation in the simpler version. The resistive film is designed tomaintain the Heaviside relationship, wherein the ratio of series toshunt resistance is equal to the ratio of the series inductance to theshunt capacitance, which is the one case where the attenuation of thetransmission line is not a function of the frequency.

SUMMARY OF THE INVENTION The attenuator element consists of aninsulating substrate supporting one or more layers of resistive filmwhich represents a lossy transmission line with distributed series andshunt losses. The resistive film may either be a symmetrical one inrespect to ground as shown in FIG. 1 of US. Pat. No. 3,157,846 or FIG. 1of U.S. Pat. No. 3,260,971 or FIG. 1 of US. Pat. No. 3,227,975. On theother hand, it also may be an unsymmetrical construction in respect toground such as is shown in FIG. 5 of US. Pat. No. 3,157,846 or FIG. 9 ofUS. Pat. No. 3,260,971. The supporting insulating substrate can be flator curved as long as the surrounding conductor is shaped to satisfy therequirements of the Heaviside relation which requires that the ratio ofseries to shunt resistance equals the ratio of series inductance toshunt capacitance. One end of this network is connected to the inputconnector through a conductive launching electrode, the other end over aconductive or capacitive contact to the sliding inner conductor grrd theoutput. A conductive contact is used if the frequency range includes DC.or very low frequencies where the series capacity of a capacitivecontact would cause frequency sensitivity of attenuation and matchvT heresistive attenuator element is enclosed inside the inner conductor and,therefore, is in side a field-free region. Only the part that is desiredin the respective attenuation position is inserted in the transmissionline by moving it out of the enclosing inner conductor. A brush orcapacitive contact connected to the movable inner conductor connects theseries film to the inner conductor, thus making the connectioninput-output through the series film. At the same time, the insertedpart of the shunt film is connected to ground through a continuousconductive or capacitive ground contact; in this case, over a series ofcontacts that are pressed down on the edge or edges of the resistor by amultiple finger spring. Adjusting the attenuator does two thingssimultaneously, first it connects a section of the series film betweeninput and output, and second at the same time, an increasing number ofbrush contacts are connecting the inserted shunt area to ground so thata variable resistive attenuator element is being inserted in thetransmission line. Since the unused part of the resistive element is ina field-free region, it is not connected in parallel with the output asin other types of variable card attenuators. This makes it possible toconnect the movable inner conductor directly to the fixed input launcherin the zero attenuation position and to decrease the minimum loss to theloss of the transmission line between input and output with theresistive element completely out of the circuit. The sliding innerconductor makes contact with a conductive output transmission lineportion, the effective length of which is varied in one version as thesliding center conductor is moved so as to maintain the total electricallength between input and output constant as will be explained below, andthus provides a constant electrical length so that no phase shift occursirrespective of attenuation value.

The specific nature of the invention, as well as other objects andadvantages thereof, will clearly appear from a description of apreferred embodiment as shown in the accompanying drawings, in which:

FIG. l is a transverse sectional view through the attenuator, taken online 1-1 of FIG. 3;

FIG. la is a schematic drawing used to explain the principle of theattenuator;

FIG. 2 is a view taken on line 22 of FIG. 1;

FIG. 3 is a view taken on line 33 of FIG. 1;

FIG. 4 is a view taken on line 44 of FIG. 2;

FIG. 5 is a bottom view of the edge contact sector of the attenuator;

FIG. 6 is an enlarged sectional view similar to that of FIG. 4, showingdetails of the spring finger contacts;

FIG. 7 is a plan view showing the slidng sleeve in place of the ceramicannulus;

FIG. 8 is a sectional view taken on line 88 of FIG. 7; and

FIG. 8a is an enlarged view of the sliding contact of FIG. 8.

Referring mainly to FIGS. 1 and 2, the attenuator is provided with ahousing 2 having in its calibrated version a dial 4 on its face providedwith appropriate markings or indicia to indicate the attenuation valuewhich is controlled by the rotational setting of knob 6 carrying aconventional pointer or arrow inscribed on its face so that therotational setting of the knob can be read in terms of the desiredparameter to express the value of attenuation in the circuit asdetermined by the knob setting. Knob 6 turns control shaft 8 whichpreferably drives attenuator shaft through a suitable backlash-freereduction gear arrangement 9 so that a nearly full turn of the knobproduces a much smaller turn of the shaft 10, since the full travel ofthe attenuator, as will be seen, is only a small arc of the entirecircumference, thus providing, in effect, an expanded scale which can bereadily read for greater accuracy.

Control shaft 10 extends through partition 12 into attenuator chamber14, where it is squared off or otherwise arranged so as to rotatablycarry attenuator elements l6, 18, 20, 22 and 23 but not the annularceramic disc 24, which is supported independently and fixed to thehousing 2 so that the ceramic disc cannot rotate within the housing. Insome cases it may, however, be more practical to move or rotate theresistor substrate and keep the hollow inner sleeve stationary, since itdoes not matter whether the sleeve or the resistor is moved so long asthere is relative motion between them.

Element 16 (FIGS. 2 and 4) is a conductive metal disc cut away aboutone-half the radial distance out from the center for about one-third ofits circumference, leaving a circular center portion 30 and an arcuatecircumferencial 32 of about 120 circumferencial extent. Sector 34,intermediate in radius between 30 and 32, extends for the remaining 120.The entire rotor assembly is fixed to the shaft 10 so that the entireassembly rotates with the shaft as a unit. This metal disc 16 serves asa portion of the grounded outer conductor.

Element 18 (FIG. 3) has a metal sector that extends for 240 and is fixedto sleeve 22 in any desired manner, shown as a thin insulating piece 23fixed to both the sleeve and element 18, and element 18 is ofsufficiently small diameter so that it fits loosely into the centralaperture of ceramic disc 24, and it also is rotated by shaft 10. Itsarcuate sector portion 18a corresponds to the arcuate sections 32 and 34of element 16.

Element (FIGS. 1, 4 and 6) also is carried by shaft 10 and at 21 has thesame shape and size as the metal portion 32 of element 16 which itoverlies. This disc serves as a portion of the grounded outer conductor.As shown in FIGS. 1 and 6, it has a downwardly extending rim 210 at itsouter circumference, and extending back radially and inwardly from therim are a number of spring fingers 38 each having a small piple 40 nearits outer end which engages a dimple cut into the top of conductingcontact block 42 so that the bottom of this block is urged into flatcontact with the inner edge of the shunt portion of resistive film 44 onceramic disc 24. This shunt film portion 44 extends from the inner edgeof the ceramic disc towards the periphery where it meets the seriesresistive portion 46 (FIGS. 2 and 6), which may be a thicker layer ofresistive material as shown in U.S. Pat. No. 3,157,846. This seriesportion 46 is connected to the center conductor of the input coaxialconnector 28 by means of a highly conducting layer 48 of suitablematerial such as gold serving as a stripline conductor (FIG. 3).However, this device can also be constructed with a single resistivefilm which serves simultaneously as series and shunt resistance.

The effective length of the variable attenuator is in a practical deviceonly a short arcuate distance of the total circumference, e.g., somewhatless than For another 120 or so the ceramic annulus is coated with asuitable conductive material such as a gold or silver strip 48 or 49providing a conductive stripline extension of the attenuator. The remoteend of this conductive stripline is connected to the inner conductor ofthe output coaxial connector 50, so that the electrical inner path ofthe attenuator is from center conductor of input coaxial connector 28'to the center conductor of coaxial connector 50.

Element 22 (FIGS. 2 and 7) is a movable arcuate center conductor whichhas an elongated U-shaped cross-section (see FIG. 8) so that it slipsover the inner edge of annular ceramic disc 24, and is a little longerthan the arcuate length of the resistive layers 44 and 46. At one end ofelement 22 is a dimple 52 which bears on a brush 53 similar to brushes42, which in turn bears on the series resistive strip 46 and at theother end it has a number of spring fingers 54 which bear resilientlyagainst the gold or silver stripline 49 which in turn is connected tothe inner conductor of coaxial connector 50.

Element 22 rotates with element 20, from which it is separated only by asmall gap. Thus, as the knob 6 is rotated to increase the attenuation,the dimple 52 moves clockwise away from the highly conductive gold strip48 into contact with the resistive center strip 46 to insert seriesattenuation into the circuit between the inner conductors of coaxialconnectors 28 and 50. At the same time the spring fingers 38 of sector21 move their contacts 42 into engagement with the inner edge of shuntresistive layer 44 to bring more and more of this shunt resistor intothe circuit between ground and inner conductor while the opposite end ofsleeve 22, through spring fingers 54, is bearing on the conductive strip49. Thus, as the resistive portion of the attenuator is inserted intothe circuit, the conductive portion represented by sleeve 22 andstripline 49 is being shortened so that the electrical length remainsthe same between the two coaxial connectors and the phase does notchange due to a change in length of the circuit as the attenuation isadded in. It will be noted that the unused portion of the resistive filmis totally shielded by sleeve 22 and so has no effect on the circuit,unlike the usual case where it constitutes a parallel branch, thoughsupposedly out of the circuit. Conversely, as the attenuation is reducedby turning the knob in the opposite direction, the sleeve 22 returns toenclose the unused portion.

The principle will perhaps be easier to understand by reference to FIG.1a which is a schematic diagram of a linear version which is not asrealistic in constructional detail and is used mainly to illustrate theprinciple of the invention. It depicts a straight-line model of theattenuator, rather than the rotary model shown in the remaining Figures,but the basic principle is the same in both cases, and the attenuator ofthe invention can be made either way. The same reference characters willbe used for corresponding parts of the straight and rotary models whereapplicable, with the subscript added for the straight-line model.

The grounded outer conductor of the attenuator is represented at 2a andcan be thought of as a tubular outer ground conductor of circular orrectangular cross section. The resistive attenuator portion is in theform of a thin film of resistive material deposited on a ceramic plate24a and having a series portion 46a and a shunt resistive portion 44a,which may be a thinner layer of resistive material than 46a. Springfingers 38a are carried by sleeve 20a and an increasing number of themmake contact with the lower edge of the shunt resistive film 44a as thesleeve 20a is moved (by means not shown) to the right. Shielding sleeve22a carrying contact 52a moves together with the sleeve 20a andmaintains contact with conductive strip 49a (which serves as aconductive stripline between the resistive portion of the attenuator andthe output connector a) by means of spring finger contacts 54a.

The linear version can also be constructed using a ceramic rod or tubeas the substrate for the resistive film, in which the series portioncorresponding to 460 is provided by a section of low resistivitydistributed over an arcuate section ofabout one-sixth to one-third ofthe circumference, and the remaining portion of the circumferenceprovides the shunt resistive portion which is wrapped around the tubeand connected to ground at the side opposite to the series portion. Theconnection to ground can be made through suitable sliding contacts orthrough a sleeve or block in close proximity which provides a capacitivecontact to ground. This construction has the advantage that the wholesurface of the ceramic substrate is used to support the resistive filmand not only one side as in the flat substrate. To provide the samesurface area for the resistor the supporting substrate can, therefore,have a smaller cross section. This in turn requires a smaller enclosingsleeve inner conductor so that the outer conductor that providesthematched enclosing ground is of smaller cross section. Since thehighest operating frequency of a TEM or quasi TEM structure isdetermined by the cross section, this construction provides a design forvery high microwave frequencies.

In FIG. 1a, the total length between input and output is divided intothree sections 11 1 and 1 11 is the length of the attenuator elementinserted in the tramsmission path. This inserted attenuator elementbehaves like a lossy transmission line with an effective dielectricconstant K The electrical length of this section is therefore, 1 I( 1the transmission length of the sliding center conductor. K the effectivedielectric constant, Vi -l, the electrical length of this section.

i transmission length of the wide stripline on which the movable centerconductor slides back and forth. K is the effective dielectric constant.1 is the electrical length of section 1 The total electrical length ofthe unit is, therefore,

VITA, VK l x l i l I is shortened by whatever amount ll is lengthened,when the attenuation is changed. The sum 1 1 is, therefore, constant.Making the effective dielectric constant of region 1 and 3 equalresults, therefore, in a constant electrical length with varyingattenuation which is W, X (1 1 :i- 1 Since this is constant, there is nophase shift when the attenuation is changed.

In a practical case it is desired to make the attenuation suitable formatching a 50 ohm circuit. The width of the stripline for 50 ohmsdetermines the size of the enclosure around it. If the stripline iswider, the ground planes must be further removed or separated; where thestripline is wide, the ground planes are provided by the top cover andthe housing, but in the region where the stripline is much narrower, theground planes must be brought closer to maintain the 50 ohm impedance.Therefore, we provide sector-shaped rotor parts in the circularconstruction with a step where we change from narrow to wide, and thisstep rotates with the arcuate member, or moves with the linear member20a in the linear version of FIG. lla. Resistive films can bemanufactured which have no frequency sensitivity from DC. to beyond 26GHz. It is therefore practical to design this device to cover a muchhigher frequency range than DC. to 8 GHz. However, since this deviceuses a quasi TEM mode in the resistive region and a TEM mode in theconductive regions, higher modes can be excited if the spacings betweengrounds are sufficiently large to permit their occurence. It will thenbe necessary to use two precautions: use such a construction as, e.g.,symmetry which will not excite a higher order mode and install modeabsorbing devices such as resistors or ferrites which will absorb theundesirable mode without interfering with the function of the device.However, by making the unit very much smaller, it is possible to use itat much higher frequencies.

It will be noted that the spacing of the ground planes above and belowthe center conductor as shown in FIG. 1 is different in FIG. 4. This isdone to maintain the impedance constant in the two cases, since theconductive film at 49 (FIG. 3) is wider than at 48, and the sleeve 22extends across all of this wide portion; it is therefore necessary tomake the spacing to the ground conductor greater in the sleeve coveredportion of the inner conductor, and this is provided by the cut-awayportions of the conductive rotary elements 16, I8, and 20. It could alsobe done by adding insulation to adjust the effective capacitance betweenground and the exposed portion of 44 and 46, but in any case, it isnecessary, in order to maintain the impedance at the desired value (inpractice, usually 50 ohms) to adjust the parameters (series inductanceand shunt capacity) of the two sections, i.e., the effective exposedattenuator section and the sleeve-covered section, and this is providedfor by making the entire variable portion, both center conductor andground conductor, of the proper configuration and movable together asshown, so that there is no change in the desired matching condition asthe attenuator is adjusted to various values, and there is no mismatchbetween the variable part of the center conductor and the rest of thecenter conductor.

I claim:

11. a. A variable resistive film attenuator of the type having agrounded outer conductor and an inner conductor with a resistive filmportion lying on an insulattive portion to a portion which can beelectrically connected to the grounded outer conductor,

0. a conductive inner conductor connecting one end of said resistivefilm to the inner conductor of an input circuit connector device,

(I. and a second inner highly conductive conductor adjacent to the otherend of said resistive film and connected to the inner conductor of anoutput circuit connector device,

e. conductive shield means movable relative to said resistive filmportion and closely surrounding the uninserted portion of the filmattenuator which remains as a part of the film is inserted into thecircuit, said shield means being electrically connected at one end tosaid second inner conductor,

. sliding contact means at the other end of said shield means inelectrical contact with said series resistive film to insert anincreasing length of said resistive film into the circuit as the shieldmeans is moved relative to the resistive film to expose more of theseries resistive film and thus to increase the attenuation,

g. and grounded slider contact means to electrically contact a variableportion of said shunt portion of the resistive film as the shield meansis moved relative to the resistive film portion and thus insert avariable portion of the resistive film in shunt between the inner andouter conductors.

2. The invention according to claim 1, wherein the length between theinput and output is physically maintained constant, for all attenuationvalues.

3. The invention according to claim 1, wherein the resistor is locatedon a flat disc.

4. The invention according to claim 1, wherein the resistor is locatedon a flat strip.

5. The invention according to claim 1, and outer ground conductor meansmovable with said shield means and having portions spaced differentlyfrom the inserted portion of the resistive film and from the shieldedportion so as to maintain the same matched impedance along the twoportions as the inserted attenuation is varied.

6. The invention according to claim 5, said grounded contact meanscomprising an array of individual conductive spring elements, eachterminating in a single contact element so that more or less of thesecontact elements are brought into contact with the shunt resistiveportion of the film as the contact means is moved to adjust theattenuator.

7. The invention according to claim 6, each said single contact elementbeing a small block of conductive material having a flat bottom portionand having on top a pivotal connection with its spring element so thatsaid bottom portion is maintained flat in contact with the attenuatorfilm portion on which it slides.

1. A. A variable resistive film attenuator of the type having a groundedouter conductor and an inner conductor with a resistive film portionlying on an insulating support element, b. said resistive film portionincluding a series resistive film portion and at least one shuntresistive film portion extending away from the series resistive portionto a portion which can be electrically connected to the grounded outerconductor, c. a conductive inner conductor connecting one end of saidresistive film to the inner conductor of an input circuit connectordevice, d. and a second inner highly conductive conductor adjacent tothe other end of said resistive film and connected to the innerconductor of an output circuit connector device, e. conductive shieldmeans movable relative to said resistive film portion and closelysurrounding the uninserted portion of the film attenuator which remainsas a part of the film is inserted into the circuit, said shield meansbeing electrically connected at one end to said second inner conductor,f. sliding contact means at the other end of said shield means inelectrical contact with said sEries resistive film to insert anincreasing length of said resistive film into the circuit as the shieldmeans is moved relative to the resistive film to expose more of theseries resistive film and thus to increase the attenuation, g. andgrounded slider contact means to electrically contact a variable portionof said shunt portion of the resistive film as the shield means is movedrelative to the resistive film portion and thus insert a variableportion of the resistive film in shunt between the inner and outerconductors.
 2. The invention according to claim 1, wherein the lengthbetween the input and output is physically maintained constant, for allattenuation values.
 3. The invention according to claim 1, wherein theresistor is located on a flat disc.
 4. The invention according to claim1, wherein the resistor is located on a flat strip.
 5. The inventionaccording to claim 1, and outer ground conductor means movable with saidshield means and having portions spaced differently from the insertedportion of the resistive film and from the shielded portion so as tomaintain the same matched impedance along the two portions as theinserted attenuation is varied.
 6. The invention according to claim 5,said grounded contact means comprising an array of individual conductivespring elements, each terminating in a single contact element so thatmore or less of these contact elements are brought into contact with theshunt resistive portion of the film as the contact means is moved toadjust the attenuator.
 7. The invention according to claim 6, each saidsingle contact element being a small block of conductive material havinga flat bottom portion and having on top a pivotal connection with itsspring element so that said bottom portion is maintained flat in contactwith the attenuator film portion on which it slides.